In many industrial chemical processes, various separation techniques are used to isolate one material from another. These separations can be liquid from liquid or liquid from solid. Two common separation mechanisms that can be useful for chemical processes, and in particular, aromatic hydrocarbon separations, include chromatography and centrifugal force.
Chromatography has many variations and can be performed on a large scale for chemical separation or on a microscale for analytical purposes. Chromatographic methods generally rely on differences in the affinities of the various members of a group of dissolved or gaseous chemicals for a certain adsorbent. Typically, all chromatographic methods have a mobile and a stationary phase. The mixture is placed in the mobile phase that is then passed through the adsorbent-containing stationary phase. The different components of the mixture have different affinities for the adsorbent in the stationary phase, and these differences in affinities result in different rates of passage through the stationary phase, resulting in the separation.
Centrifuging is commonly used for separating solids from liquids (where one of the materials to be separated can be solidified) and for liquid from liquid mixtures. Centrifuging generally utilizes a centrifuge device that spins its contents either vertically or horizontally to increase the normal effect of gravity. In a rotating centrifuge, the denser particles will generally move to the outside of a cylinder, while the lighter particles remain near the center of that cylinder.
For many chemical processes, such solid-liquid separation methods often play an important role in the isolation and manufacture of intermediate chemical streams. The separation of aromatics, and in particular, xylene isomers are quite suitable due to advances in crystallization technology, which permits a chemical plant operator to crystallize discrete xylene isomers from a mixture of xylene isomers. Crystallization combined with efficient solid-liquid separation techniques is of interest because of the usefulness of paraxylene in the manufacture of terephthalic acid, an intermediate in the manufacture of polyester. Specifically, paraxylene having a purity of at least about 99 weight percent, more preferably at least about 99.5 weight percent, more preferably of at least about 99.7 weight percent, is most suitable for the manufacture of terephthalic acid by the oxidation of paraxylene.
Current commercial processes for separating xylene isomers include the aforementioned chromatography, and crystallization followed by centrifugation. Crystallization, rather than distillation, is typically a more suitable option to separate xylene isomers due to the fact that their respective freezing points are far apart, while their boiling points are in close proximity. For example, pure paraxylene freezes at 56° F., pure metaxylene freezes at −54° F., pure orthoxylene freezes at −13° F., and pure ethylbenzene freezes at −139° F. Equilibrium mixtures of xylene isomers generally contain about 25 weight percent paraxylene, about 25 weight percent orthoxylene, and about 50 weight percent metaxylene.
Due to the low concentration of paraxylene in these mixed xylene streams and the disparate freezing points of the xylene isomers, very low temperatures are generally required to ensure maximum recovery of paraxylene from a C8 fraction by crystallization. However, there is an operational low temperature limit generally taken as the metaxylene/paraxylene or the orthoxylene/paraxylene binary eutectic temperature that prevents the complete recovery of all the paraxylene from a C8 fraction.
At or below this limit, either metaxylene or orthoxylene will co-crystallize with paraxylene. Furthermore, if the temperature falls below either of the binary eutectic temperatures, then a second solid phase which is lean in paraxylene will crystallize from the mixture. The formation of a second solid phase is generally viewed as undesirable, so crystallization processes are typically operated at as cold a temperature as possible, but at a temperature warmer than the warmest binary eutectic temperature. While this constrains the once-through paraxylene recovery of the process, conventional paraxylene separation processes that use crystallization produce a substantially pure paraxylene product.
Although such crystallization processes produce a paraxylene product with a purity level in excess of 98 percent, the use of centrifuges, centrifuge-like devices, and other solid-liquid separation devices can add significant costs to the purification process due to their high capital costs and the high maintenance costs inherent in high speed rotating parts. In addition, such devices are expensive to buy, install, operate, and maintain. They are also a reliability problem since even well-maintained centrifuges are apt to shut down unexpectedly. As a result, prior efforts have focused on developing alternatives to centrifugation to improve the economics of producing substantially pure paraxylene.
U.S. Pat. Nos. 4,734,102 and 4,735,781, issued to Thijssen et al. disclose solid-liquid separation processes and apparatuses that function with minimal moving parts. The process and apparatus of Thijssen '102 and '781 utilize a closed column having at least one filter tube having a filter. A suspension is directed into one end of the column, and a washing liquid into a second end of the column in countercurrent flow to the suspension, forming a bed in the column. A filtrate stream from the suspension is removed through the filters of the filter tubes into the interior of the tubes, and a concentrated suspension is withdrawn from the second end of the column. A wash liquid is introduced at the second end to wash and reslurry the concentrated suspension, When the process is used to separate a suspension derived from a melt crystallization process, the wash liquid comprises molten crystal product from the suspension.
Although the processes and apparatuses disclosed in Thijssen '102 and '781 avoid centrifuging, these processes have disadvantages that have limited their broad application.
The process disclosed in the Thijssen patents cannot effectively separate liquids from solids at processing temperatures far below the melting point of slurry crystals derived from a melt crystallization process. This is because the wash liquid utilized during the process freezes within the column during the washing part of the process. At increasingly lower temperatures, the freezing wash liquid fills a larger portion of the void fraction between the solids, thereby requiring higher and higher pressures to drive the wash liquid into the column. Eventually, a low enough temperature will be reached wherein the freezing wash liquid essentially plugs the device, causing failure and imminent shutdown of the apparatus and process disclosed in the Thijssen patents. In the case of the separation of paraxylene from xylene isomers, the application of this technology would preclude the manufacturer from operating its crystallization process at aggressively low crystallization temperatures so as to maximize paraxylene recovery by challenging the binary eutectic temperatures described above.
Yet another disadvantage is that the use of a molten solids wash liquid in the process disclosed in the Thijssen patents can contaminate the filtrate with a liquid that may not be easily or inexpensively separated from the filtrate. This can result in a substantial loss of solid product to the filtrate.
Consequently, there is still a great need in the industry for alternative processes and apparatuses for separation of solids from liquids that address and solve the problems noted above.
It has now been found that filter column apparatuses in accordance with the present invention and comprising a filtration zone and a reslurry zone substantially separated by a barrier wall provides substantial energy and capital savings benefits over apparatuses that do not feature such a barrier wall.
It has also been found that a filter column apparatus comprising a filtration zone and a reslurry zone in substantial cooperation with one another provides for substantial energy and capital savings over apparatuses where reslurry operations occur in separate downstream vessels.
It has also been found that processes for separating at least a portion of one or more substantially solid components from a solid-liquid stream, in a filtration zone, by contacting at least a portion of the substantially solid components and/or the solid-liquid stream with an immiscible fluid, such as a gas, produces a relatively dry and pure product stream of substantially solid components.
It has also been found that processes for purifying paraxylene from a solid-liquid stream having a wide range of temperatures, in a filtration zone, by contacting at least a portion of either substantially solid paraxylene or said solid-liquid stream with an immiscible fluid, such as a gas, in lieu of a wash liquid, produces a relatively dry and pure product stream comprising substantially solid paraxylene, which can be further processed with little or no additional refrigeration costs.